Set image size ONLY for editor

I’m creating a theme which takes away any image resizing done by WordPress and uses CSS to dynamically change image sizes with % widths. This means when an image is uploaded to the editor, it needs to be full-sized – but this makes editing posts a pain if there’s a huge image taking up lots of editing space.

So I’m wondering if there’s some way I can add in a function to change the way the images display in the editor but not actually permanently alter the image size?